Heat Recovery Steam Generators (HRSGs) include evaporator tube rows (multiple tube rows are commonly referred to as tube bundles) that transfer heat from an exhaust-gas stream, such as that from a combustion turbine or other industrial process that produces hot gas, to a fluid inside the evaporator tubes. Horizontal HRSGs employ vertical evaporator tube rows arranged in cross-flow to an exhaust-gas stream that flows in a horizontal direction across the vertical evaporator tubes. An evaporator section on HRSGs typically includes lower manifolds (headers) to distribute water to the bottom of the evaporator tubes, and upper manifolds (headers) to collect a mixture of steam and water from the top of the evaporator tubes.
One type of horizontal HRSG is a circulation type horizontal HRSG. In such HRSGs, circulating fluid is only partly evaporated when passing through evaporator tubes. The fluid inside the evaporator tubes never becomes superheated because an excess mass flow of fluid is maintained at all times. For this reason, the temperature of the fluid inside the evaporator tubes of circulation type horizontal HRSGs is essentially constant. The fluid that is not evaporated in the process is fed again to the same evaporator tubes for further evaporation after separation of generated steam in a steam drum.
Walls of a steam drum in a circulation type horizontal HRSG are subjected to large thermal stresses when the steam drum is rapidly heated. Repeated heating and cooling reduces the life of the steam drum, leading to eventual failure of the circulation type horizontal HRSG. To avoid steam drum failure, operating restrictions are typically imposed on circulation type horizontal HRSGs to reduce the rate of warm-up of the steam drum.
Another type of horizontal HRSG is a once-through horizontal HRSG. This type horizontal HRSG lacks a steam drum, thus operating restrictions to avoid rapid warm-up are not necessary. Further, a once-through type horizontal HRSG is not subject to any pressure limitation. Therefore, live-steam pressures well above the critical pressure of water (Pcri=221 bar), where there is only a slight difference in density between a medium similar to a liquid and a medium similar to steam, are possible. A high live-steam pressure promotes a high thermal efficiency and thus low CO2 emissions of a fossil-fired power station. Fluid fed through a once-through HRSG is completely evaporated in a single pass through either a single heating area, or a plurality of heating areas connected in series.
In addition, a once-through type horizontal HRSG has a simple construction compared with that of a circulation type horizontal HRSG, and can therefore be manufactured at an especially low cost compared to the manufacture of a circulation type horizontal HRSG. Further, a once-through type horizontal HRSG, in contrast to a once-through type vertical HRSG, can be manufactured especially simply and at an especially low production and assembly cost.
Common to all horizontal HRSGs, the temperature of the exhaust-gas stream declines from the exhaust-gas inlet to the exhaust-gas outlet of the evaporator section. The amount of heat transferred in each tube row over which the exhaust-gas flows is proportional to the temperature difference between the exhaust-gas and the fluid in the tubes. Therefore, for each successive row of evaporator tubes in the direction of exhaust-gas flow, a smaller amount of heat is transferred, and the heat flux from the exhaust-gas to the fluid inside the tube declines with each tube row from the inlet to the outlet of the evaporator section.
Geodetic pressure drop describes the pressure drop due to the weight of the water column and steam column relative to the area of a cross-section of a flow medium in a steam-generator tube. Friction pressure loss describes the pressure drop in a steam-generator tube as a result of the flow resistance for the flow medium. The total pressure drop in a steam-generator tube is essentially composed of the geodetic pressure drop and the friction pressure loss.
During especially intense heating of an individual steam-generator tube, the steam generation in the steam-generator tube becomes especially high. The weight of the flow medium that has not evaporated in the steam-generator tube therefore decreases, so that the geodetic pressure drop in the steam-generator tube likewise decreases. However, in a once-through type steam generator, all steam-generator tubes are connected in parallel inside a once-through heating area. Each of these parallel tubes have the same total pressure drop on account of their common connection to a flow medium inlet and their common connection to a flow medium discharge. If there is a geodetic pressure drop in one of the parallel steam-generator tubes that is especially low compared with the other steam-generator tubes connected in parallel with it, on account of different heat intensity, an especially large quantity of flow medium then flows for a pressure balance through the tube heated to a greater degree if the geodetic pressure drop is on average the dominant portion of the total pressure drop on account of the configuration of a once-through heating area.
In other words, a steam-generator tube heated more intensely, compared with steam-generator tubes connected in parallel with it, has an increased flow rate of a flow medium. On the other hand, a steam-generator tube heated to an especially low degree compared with other steam-generator tubes connected in parallel with it has an especially low flow rate of flow medium. By a suitable specification of the ratio of friction pressure loss to geodetic pressure drop due to the configuration of the steam-generator tubes, in particular with regard to the selected mass-flow density in the steam-generator tubes, this effect can be utilized for automatic adaptation of the flow rate of each steam-generator tube to its heating.
A once-through type horizontal HRSG that compensates for this difference in flow rate is known. However, in all once-through type horizontal HRSGs, including that accounting for pressure differences, the temperature of steam-generator tube metal is determined by both the amount of heat flux across the steam-generator tube wall and the average temperature of the flow medium inside the steam-generator tube. Since the heat flux declines from the inlet to the outlet of the evaporator section, the temperature of the steam-generator tube metal is different for each row of steam-generator tubes included in the evaporator section.
Each manifold (header) of a horizontal HRSG that runs perpendicular to the exhaust-gas flow acts as a collection point for multiple rows of tubes. These headers are of relatively large diameter and thickness to accommodate the multiple tube rows. FIGS. 1a and 1b are two views of such an assembly 100, known as a multi-row header-and-tube assembly, utilized in once-through type horizontal HRSG that compensates for pressure differences. Included in the assembly 100 is a header 101 and multiple tube rows 105A–105C. As shown in FIG. 1a, each individual tube row 105A–105C includes multiple tubes. In the interest of clarity of illustration, FIG. 1b only shows a single tube in each tube row 105A–105C. Since each of tube rows 105A–105C is at a different temperature, the mechanical force due to thermal expansion is different for each tube row 105A–105C. Such differential thermal expansion causes stress at tube bends and the attachment point of each individual tube to the header 101. Further, also contributing to thermal stresses at the attachment point of each individual tube to the header 101 is a difference in thickness between the relatively thin-wall tubes as compared to the thick-wall header 101. Under certain operating conditions, these stresses can cause failure of the attachment point, especially if the assembly 100 is subjected to many cycles of heating and cooling.
Thus, while a once-through type horizontal HRSG that both compensates for pressure differentials in steam-generator tubes and lacks a steam drum is known, it is nonetheless subject to failure due to thermal stresses in other components, especially in a multi-row header-and-tube assembly 100. Accordingly, a need exists for a once-through horizontal HRSG that is capable of both rapid heating and cooling as well as a large number of start-stop cycles.